Of course that made a lot more sense in the 18th century than it does in the 21st, we tend to view states that routinely engage in violent coups as failures, and the man who famously said of Shay's Rebellion that "the tree of liberty must from time to time be refreshed with the blood of tyrants and patriots" ran from Monticello like a coward when the British showed up and left its defense to his slaves.
But the last line of defense is supposed to be "we just kill them all." What the founders didn't anticipate was that the militias would side with the tyrants, or that society and its morals would advance to the point where political violence was no longer seen as a noble act except by extremists.
They may not have been able to foresee the sheer levels of corruption we have today, but they should have been able to see that in trying to keep Congress from becoming too powerful, they made it too weak to do its job. The fact that Congress has hundreds of members, plus the veto in the executive branch, plus the Supreme Court being able to overturn legislation, was plenty to keep it in line.
It was inevitable that the executive branch would end up taking over a lot of their functions.